Automatic light control



June 14, 1949. w. D. FLEMING AUTOMATIC LIGHT CONTROL Filed Aug. 25, 1946 I I Avg/aura? 1441.1. 04M 0. FLEMING Patented June 14, 1949 UNITED STATES PATENT OFFICE Application August 23, 194.6, Serial No. 692,420

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 1 Claim.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to automatic control devices, and more particularly to devices for the automatic control of a light source.

Although I will describe and illustrate my invention as used in connection with an automatic light control for photographic equipment, it is understood that my control device may be utilized for many other difierent purposes.

It is an object of my invention to provide a device which will automatically control the length of time that a light source is permitted to illuminate a surface, such control being accomplished by the measurement of the intensity of the light on the illuminated surface.

It is another object of my invention to provide an automatic control device that will be responsive to light reflected from an illuminated surface during exposure to a light source.

It is still another object of my invention to provide an automatic light control means for a photographic projection printer, which will automatically cause the printing light to be out 01f at the proper time to secure correct exposure of the printing paper.

The above and other objections and advantages will become readily apparent when the following specification is read in conjunction with the accompanying drawing in which the single figure is a schematic diagram of one embodiment of my invention.

In general, the time control of a light source is accomplished in my invention by causing the light which impinges upon an illuminated surface to control an electronic relay system activated by a photo-electric tube so that when the illuminated surface has received a predetermined quantity of light the illuminating source is automatically extinguished. This control is obtained by a means of a photo-electric tube which receives light from the illuminated surface during exposure to the light source. Light is received from the illuminated surface by means of a lens so placed as to produce an image of the illuminated surface or a proper portion thereof upon the cathode of the photo-tube. Referring to the drawing, it is seen that the numeral it] refers to a surface which is to be illuminated for a certain predetermined period of time. The surface l may be a sheet of sensitized photo printing paper as used in the projection printing process. The surface [0 is to be illuminated by a light source H which is inclosed in a housing l2 having a projection lens (3 in the front portion thereof. For control of the light source H a relay M is connected in the plate circuit of an electron control tube l5 and is activated by the plate current of that tube. The relay M is connected such that it closes a power circuit to the light source H by closing of armature A and contact C when the plate current of the electron control tube I5 does not flow, and opens the light circuit when plate current of electron tube l5 does flow. Relay is a two-position type which, when tie-energized, has its armature A in contact with point C.

The plate current of the electron tube I5 is controlled by the voltage on the control grid CG of that tube. A second electron tube I! which is used as a rectifier has its output connected through a switch Hi to the control grid of the electron tube 15. Switch 18 should be of a push button type or equivalent. When the switch I6 is closed, the control grid CG of tube 15 is made strongly negative by the rectified plate current of the tube I! and the negative charge is stored in a condenser Hi. The negative swing of the control grid CG of tube l5 blocks the tube and stops the plate current of that tube causing the relay I4 to become de-energized, thereby closing the circuit for the light source H.

Closing of the switch It simultaneously charges a bank of condensers IS with the same negative charge impressed on the control grid CG of the tube IS. The closing of switch It also results in the opening of the rectifier cathode circuit at AB and the closing of the light circuit through the contact of relay armature A with C, thus preventing the continual charging of condensers Ill and I9. The number of condensers utilized in the condenser bank I9 is controlled by a switch S. If desired, a single capacitor may be substituted for the condenser bank It. This charge on the condenser bank l9 holds the control grid CG of tube I 5 negative until the charge in the condenser bank IS has leaked off. Time control is obtained by the time required for the negative charge to leak from the condenser bank I 9 through a series of resistances shunted across the condenser bank 19. The resistance mentioned may comprise a pair of variable resistors 20 and 2|, as well as a fixed resistor 22. Also included in the series re' sistance is a photo tube 23 having a cathode 24 and an anode 25. The photo-electric tube 23 is inclosed in a light-tight housing 25 having a lens 21 inserted in the front portion thereof. Thus when leakage from the condenser bank I9 is sufficient for the control grid CG of the tube IE to as follows. -Jment circuit of the tubes 15 and ii. As soon as swing positive, plate current again flows in tube I5 which energizes the relay l4 opening contacts B and C and, breaking the circuit to the light source Ii.

Since the resistance of the photo-electric tube 23 is an inverse function of the light falling on its cathode 24 from the illuminated surface IE, a greater intensity of light on the surface ill will lessen the resistance of the photo tube 23. The less the resistance caused by the photo tube -23, the faster the charge leaks from the condenser bank I9 and the total exposure of surface It by the light source H is thereby lessened. With a lesser intensity of illumination on the surface Ill, the resistance of the photo tube'23 is greater, with the result that the charge on the condenser bank I!) leaks off more slowly and the relay it holds the illuminating source circuit closed a greater length of time.

Application of this operation of time control of a 5 light source to the control of photographic exposure in a projection printer is obvious. With a denser-negative the photo tube will receive less lightfrom'the surface of the paper during print- -ing and the light will be kept lit longer. With a thinner negative, exposure will be correspondingly and automatically shortened. Adjustment of this "timefunction of the photo cell to the printing rspeedpf the particular paper being used, the intensity and color of the light source, developer "factor and other variables is effected by adjust- "ment of the variable resistors 2t and 2! in series with the photo tube and by varying the capacity "of condenser bank H! by the switch S. A switch 28- allows thecontrolled light to be turned on *independently of the relay circuit. convenience as for example in focusing the pro *J'ectionprinter.

This is a -'-As stated before, the photo-electric tube 23 '-is inolosed in a light-tight housing 26, having aconvex lens 21 in an opening in the front of the housing, The lens 21 is focused on the cathode 24 "of the photo-electric tube. The entire photo-elec- "tric unit is so positioned as to receive light from the illuminated surface If! to image a convenient rareaof that surface onto the cathode 24 of the photo tube. Since the lens '2'! serves only to concentrate the light from the illuminated surface To secure sthe vacuum type and when used with a projection -printer,-such as illustrated here it should have a; t

l a normally-closed contact in circuit with said light cathode sensitivity corresponding to the spectral sensitivity. of the photographic material used.

A complete cycle of operation of the device is The power switch P closes the filalthese tubes warm up plate current flows in tube i.i5...energizing.the relay M which breaks the controlled light circuit and closes the cathode circuit -ofatube 'l'l thereby letting it rectify the line cur- On closing :rent. and charge the condenser I55. (the-switch IS the rectified voltage from tube H and the .charge on condenser i 8 swing the control :ggrid CG of .tube I 5 strongly negative, stopping the plate;:current flow in tube I 5, which stoppage ii. In the example :illustrated. this will start the exposure of the printin -paper in theprojection printer, When (relay llis-de-energized it also opens contacts A the cathode circuit of tube I! remains open until the charge .-on the condenser bank I9 leaks off through resistors 20, 2|, 22 and the photo tube.

When the charge on the condenser bank l9 has leaked off, the control grid of tube l5 goes positive again and plate current in tube l5 flows thus energizing the relay l4. Closing of relay M breaks the controlled light circuit and restores the unit to'the initial stage ready for another operating cycle.

In the modificationillustrated in the drawing the electric light for a -projection printer is to be controlled.

The phototube unit shouldbe fixed in a position above the printing paper so that it will clear the projection lens I 3 -and the optical path ofthat lens tothe prihting-paper so that the lens 2? will image a convenient area of the-printing paper being exposed upon -the cathode-ti lof the photo tube.

The photo. tube unit may be held in position by a fixedbracket and when once focusedmay be held in that position for'all uses thereafter.

While Iflhave' desc'ribed my automatic light control inause with aprojection printer, it is obvious that -my invention-may be utilized in many other instances where an automatic light control may be" desirable."- Theillustration given is to be considered explanatoryand not limitative. Many-changes and-modifications may be made in my invention-without departing from the scope thereof as set forth'in the appended claim.

' I claim:

An automatic light controlhaving in combination a light-source illuminating an object, a powerline'for said light source, and a control means for'energizing and-deenergizing said light source, said control means comprising a photo- I electric device responsive-to the degree'of illumination of said object, a vacuum control tube having at least: an anode; a cathode and a grid, a

rectifier having a=cathode terminal and an anode terminal, a relay in the anode circuit of said control tube, said relay including an armature, a normally-open contact in its pickup'circuit and source means connecting said armature to one :side of said power line, means connecting said normally-closed contact to one terminal of said light source, means connecting said other'termtnal of said light source to the other side of said power line, means connecting said normally-open contact of said relay:tos'aidcathode'terminal of said rectifier; a first condenser connected between the anode terminal of said rectifier and said other side of the power line; a second condenser connected between said grid and cathode of the control tube; a connection between said other side of'the power line and'the cathode of the control tube; a push-button switch connected between the grid of the control tube and said anode terminal of said rectifier; and means connecting-saidphotoelectric device across said second-'condenser;' wherebyupon application of p'ower line -'voltagegasaidcontrol tube will con- 5 6 duct, causing the relay to operate opening the light circuit and through the action of the recti- UNITED STATES PATENTS fier charge said first condenser setting up the Number Name Date circuit for operation, and upon depressing said 1, 73,468 Denis Sept. 11, 1934 switch the said second condenser charges, block- 5 2,000,589 Fuller May 7, 1935 ing the control tube thereby deenergizing said 2,232,373 Dorst Feb. 18, 1941 relay and energizing said light source, illuminat- 2,274,158 Panther Feb. 24, 1942 ing said object, the photoelectric device discharg- OTHER REFERENCES ing said second condenser thereby unlocking said v control tube after a predetermined time, and 10 Publication-F01 e fl gmp er n y deenergizing said light source. y D nald nk. pu ish d in l WILLIAM D. FLEMING. June 1937. pp- -2 REFERENCES CITED The following referenlces are of record in the 15 file of this patent: 

